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Chapter 3 of 6 · Logical Reasoning
Logical Deduction
Syllogisms test whether a conclusion validly follows from given premises; use Venn diagrams for all/some/none statements and never bring outside knowledge.
Logical Deduction is a Logical Reasoning chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 9-MCQ Logical Reasoning section. Mastering the core concepts below typically secures the full chapter weightage.
The four standard syllogism statements
- A (universal affirmative): All A are B.
- E (universal negative): No A are B.
- I (particular affirmative): Some A are B.
- O (particular negative): Some A are not B.
UHS deduction MCQs give two such premises and ask which conclusions follow. The only valid technique is to draw Venn diagrams; any conclusion that holds in every possible diagram is valid, and any conclusion that fails in even one diagram is invalid.
Worked example 1
Premises: All cats are mammals. All mammals are animals. Conclusion: All cats are animals. Draw three nested circles: cats inside mammals inside animals. The conclusion holds in the only possible diagram, so it is valid.
Worked example 2 — the classic trap
Premises: Some doctors are surgeons. All surgeons are skilled. Conclusion 1: Some doctors are skilled. Conclusion 2: All doctors are skilled.
Draw circles: surgeons sit entirely inside skilled. The doctors circle overlaps with surgeons. The overlap part of doctors is therefore inside skilled, so "some doctors are skilled" is forced. But the non-overlap part of doctors may or may not be skilled, so "all doctors are skilled" is not forced. Conclusion 1 follows; Conclusion 2 does not.
Standard validity rules to memorise
- From two universal negatives (No A are B; No B are C), no conclusion follows.
- From two particulars (Some A are B; Some B are C), no conclusion follows.
- From All A are B and All B are C, only "All A are C" and "Some C are A" follow.
- The conclusion can never be more universal than the weakest premise. From a "some", you can never deduce an "all".
Common trap: outside knowledge
If a premise says "All birds can fly", accept it for the question even though penguins exist. Logical deduction is purely formal; real-world truth is irrelevant.
Either-or conclusions
When two conclusions are individually invalid but cover all cases between them, the answer is "either conclusion 1 or conclusion 2 follows". Example: "Some A are B" or "No A are B" — one must be true; together they exhaust the possibilities.
Key Concepts
- Syllogisms
- All/some/no statements
- Venn diagrams
- Valid vs invalid conclusions
- Quick elimination
Worked MCQs
Q1. Premises: All doctors are educated. Some educated people are wealthy. Which conclusion follows?
- A. All doctors are wealthy.
- B. Some doctors are wealthy.
- C. No doctors are wealthy.
- D. None of the above necessarily follows. ✓
Explanation: The wealthy overlap may or may not include the doctors. From a 'some' premise about educated people, we cannot force any conclusion about doctors specifically.
Common trap: Common trap: students assume that because doctors are educated and some educated are wealthy, some doctors must be wealthy. The overlap might miss the doctors entirely.
Q2. Premises: No reptiles are mammals. All snakes are reptiles. Which conclusion follows?
- A. All snakes are mammals.
- B. Some snakes are mammals.
- C. No snakes are mammals. ✓
- D. Some mammals are snakes.
Explanation: Snakes sit inside reptiles, and reptiles are entirely outside mammals. Therefore no snakes are mammals.
Frequently Asked Questions
Should I use Venn diagrams or formula-style rules?
Venn diagrams. They handle every case correctly. Memorising rules without diagrams leads to errors on tricky items.
Can a conclusion be 'partly true'?
No. In formal deduction, a conclusion either follows in every possible diagram or it does not follow at all. There is no middle ground.
What if the premise is factually wrong?
Accept it for the question. Deduction tests formal validity, not real-world truth. 'All birds swim' is treated as true if stated.
How many deduction items per UHS paper?
Typically 1 to 2 MCQs. They are reliable points if you have practiced Venn diagrams.
How Logical Deduction Is Tested
MDCAT questions on Logical Deduction are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.
Practice
Drill Logical Deduction and the rest of Logical Reasoning — free, no signup.
See the full MDCAT 2026 syllabus or browse all Logical Reasoning chapters.